1. Field of the Invention
This invention relates to frequency counters and more particularly to frequency counters having increased amplifier input gain and the ability to detect and distinguish a dominant signal of interest from a signal resulting from amplifier self-oscillation.
2. Description of the Prior Art
A frequency counter is a test instrument which measures the frequency of an electrical signal. The frequency is typically determined by counting the number of times the oscillating signal crosses some voltage, usually a zero volt reference, during some precise time interval.
A typical prior art counter is shown in FIG. 1 within the dotted lines with reference number 4. An electrical signal is presented to the input through input circuitry 10 containing an amplifier which amplifies the input signal to a level which can be acted upon by the electronic components of the prior art counter.
An additional function of the input circuitry 10 is that it must accept any electrical input signal such as a sine wave, square wave, pulses or a complex signal which may be a combination of such signals including signals of varying polarity, amplitude or frequency and produce at the output one electrical pulse per input cycle. For this output pulse to be used by the subsequent electronic circuitry, the output pulse must be of a constant amplitude and width for any input signal. This conversion from varying input signals to a string of output pulses is usually done by a Schmitt-trigger circuit, such as is common in the art.
The output pulse from the input circuitry 10 is passed to a main gate 6. Conceptually, the main gate is nothing more than an on-off switch. When the switch is closed the pulses pass to a main counter 7. When main gate 6 is open, no signal is passed to the main counter 7.
Main gate 6 is controlled by a time base 5 which tells the gate when to open and when to close. Time base 5 is usually a clock which generates accurately spaced electrical pulses which open and close main gate 6. Typically, time base 5 produces gate open times in decade steps from as short as 1 microsecond to more than 10 seconds.
A gate light 11 indicates, usually through a light on the front panel of the counter 4, that the main gate 6 is closed and thereby allowing the pulses generated by the Schmitt-trigger circuitry to pass to the main counter 7.
The main counter 7 counts the number of pulses passed through main gate 6. In a typical prior art frequency counter 4, the time base 5 closes the main gate 6 for a period of one second. Therefore, if a pulse is generated for each cycle of the input signal by the Schmitt-trigger circuitry, the number of pulses counted by the main counter 7 in one second is equal to the frequency of the input signal since frequency is defined as the number of cycles per second.
In the typical prior art frequency counter, a display driver 8 is connected to the main counter 7 to convert the electrical high level signals, on the main counter circuitry representing the number of pulses counted, into low level electrical signals for driving a frequency display 9. Display 9 may include columnar gas read outs such as neon lamps or incandescent lamps, in-line-to read outs such as light emitting diodes, liquid crystal display, or a meter-movement type of read out. In the typical prior art frequency counter where the time base 5 closes the main gate 6 for a period of one second, the display, which is the frequency of the input signal, is just on the number of pulses counted by the main counter 7 while the main gate 6 is closed.
Once a frequency has been determined, an electronic latch allows display driver 8 to display the frequency on display 9 for a predetermined amount of time. During the time the frequency is being displayed, a new frequency measurement may be taking place so that when the time allowed by the latch to display the last measured frequency has expired, the newly measured frequency may be allowed by the latch to be displayed.
With the advent of the microprocessor, frequency counters containing microprocessors have become more common. In such frequency counters, the time base 5, instead of being constrained to close the main gate 6 for a period of one second or decade multiples thereof, opens main gate 6 for periods of varying durations as directed by the microprocessor. While the main gate 6 is closed, main counter 7 counts the number of pulses passing through the main gate 6. Thereafter, the frequency is arithmetically calculated in the microprocessor by dividing the number of pulses counted by main counter 7 by the length of time main gate 6 is closed.
The use of a microprocessor allows for a longer or shorter time to count the pulses generated by the input signals than is available in the typical prior art counter with the decade periods of measurement. When the period that main gate 6 is closed is increased, the accuracy of the frequency counter 4 is increased. However, because main gate 6 is closed for a longer time before a frequency is determined and consequently displayed on display 9, it is often useful to initially reduce the period that main gate 6 is closed in order to get a virtually instantaneous, although less accurate frequency display. Then, if desired, a more accurate subsequent determination of the frequency can be obtained by closing main gate 6 for a longer time.
Often, the input signal strengths of high frequency signals, such as ambient radio or high frequency signals, are quite small, typically 10-15 millivolts. Moreover, the ambient signals picked up from an antenna are not only low in level, but often pick up more than one signal at a time. In any event, these input signal strengths must be amplified to an appropriate level to allow these low level signals to trigger the digital circuitry of the counter and allow the subsequent electronic components to determine the frequency of the signal. This amplification is performed by input amplifiers which amplify the input signal to an appropriate signal strength level. These amplifiers are typically limited in the amount of amplification they can perform because once the amplifier gain is increased beyond about 25 dB, the amplifiers become unstable and start self-oscillating. Amplification above 50 dB produces very strong self-oscillation while the amplification of the signal of interest drops off. When this much amplification is used, the self-produced electrical noise present in all electronic circuitry becomes more apparent. The noise of the first stage of the amplifier is amplified by subsequent stages until it is itself a countable signal. The electrical noise appears as a self-oscillation, producing a diffused, broad bandwidth signal that cannot be easily measured. These additional signals compete with the main signals of interest picked up from the antenna and may even be present in the case of no antenna input. As a result, the frequency counter displays random readings over a range of frequencies when trying to determine the frequency of the main signal. Because the frequency counter must have pure signals to accurately count the frequency, more than one signal only confuses the counting and yields incorrect measurements.
This self-oscillation is manifest in the creation of a relatively strong signal at a frequency which varies considerably with time within a predictable frequency range. Studies of amplifier self-oscillation have indicated that the range of frequencies through which an amplifier may self-oscillate is predictable, but the exact frequency of self-oscillation that is produced at a given time is unpredictable. However, studies have also found that the exact frequency of the signal produced by amplifier self-oscillation will vary with time. As a result, consecutive measures of the amplifier self-oscillation signal frequency will produce different frequencies. This self-oscillation signal is produced while the amplifier continues to amplify the signal of interest.
By contrast, the frequency of the signal of interest will remain approximately constant through repeated measurements.
The self-oscillation signal produces serious difficulties for the frequency counter. The frequency counter has difficulty in determining whether the frequency counted is the signal of interest or is the amplifier self-oscillation signal. In fact, even in the absence of a signal of interest, the counter may measure and display the frequency of the amplifier self-oscillation signal. As a result, in order to avoid creating an amplifier self-oscillation signal, amplifiers have been limited in amplification to about 25 dB. Consequently, amplifier input gain is limited to this amount. This is a problem in want of a solution.
An additional problem with the prior art frequency counters occurs when a pick-up antenna is used on the input to the amplifier. In this case, many radio frequency signals may be present at the same time along with an amplifier self-oscillation signal. These signals will combine as described in a Fourier Series to produce a resultant signal which does not resemble any of the input signals. When the counter counts the number of zero crossings per unit time from such a combined signal, the resulting calculated frequency will not accurately reflect the frequency of any of the input signals.
The exception to this is where one of the input signals is so strong that it dominates the other input signals. Such a dominant signal will typically be 10-15 dB larger than any other signal. However, such dominant signal strengths are typically found in only the near field of a transmitting transmitter. Finding such dominant signal strengths indicates that the receiver is very near to the transmitter.
In the prior art, in order to detect the presence of a nearby RF transmission, a spectrum analyzer is used. Such spectrum analyzers typically scan a RF spectrum and indicate graphically by a "spike" the signal strength of each signal found at its corresponding frequency. The height of the "spike" indicates the signal strength. Thereafter, a trained technician analyzes the resulting chart of frequency and signal strength to determine which signals are inappropriately large or unexplained, thereby indicating the presence of a transmitting device. The disadvantage of such spectrum analyzers is that they are relatively expensive, typically costing from $20,000-$30,000. Another problem with spectrum analyzers is that they are typically quite large in size. As such, they are cumbersome, bulky and difficult to transport.
Spectrum analyzers have been used in the past for security purposes to scan a large RF spectrum to ensure that no near RF transmissions anywhere in the scanned RF spectrum are present. The spectrum analyzer will produce "spikes" at all the frequencies where signals are detected, but will not distinguish between a near field transmission as opposed to a transmission of from farther away. Such a nearby transmission would merely appear as one of the "spikes" on the spectrum analyzers output which would then require a trained operator to recognize.